1. Field of the Invention
The present invention relates to a process and an apparatus for epitaxially coating a semiconductor wafer, and to an epitaxially coated semiconductor wafer.
2. The Prior Art
In semiconductor technology, crystal growth from the vapor phase is used in particular to produce epitaxially coated semiconductor wafers. The term epitaxy is understood as meaning the growth of a single-crystalline layer onto the planar boundary surface of a single-crystalline substrate, generally a substrate wafer, for example a semiconductor wafer. This coating or deposition takes place by means of what is known as chemical vapor deposition (CVD) in CVD reactors, as described for example in EP 714 998 A2. The semiconductor wafer is first of all heated by means of heating sources and then exposed to a gas mixture, referred to below as the process gas, comprising a source gas, a carrier gas and if appropriate a doping gas.
One of the most important applications is the deposition of homoepitaxial layers on single-crystalline silicon substrates, generally silicon wafers. The source gas used is, for example, silanes, such as trichlorosilane, and the carrier gas used is, for example, hydrogen. The doping gases used to dope the epitaxial layer are gaseous compounds, for example from main groups III or V of the periodic system. These compounds, for example phosphine or diborane, decompose, as does the source gas, in the vicinity of the heated wafer. The foreign atoms are then incorporated in the crystal lattice of the epitaxial layer. The semiconductor wafer (substrate wafer) and epitaxial layer are generally differently doped, in order to obtain a sudden transition in the electrical properties. For example, there can be a steep rise in the resistance profile at the transition from the substrate wafer to the epitaxial layer.
During the deposition of the epitaxial layer, there is an undesirable escape of elements which were used for doping the substrate (substrate dopants, for example, in the case of silicon substrates, boron, arsenic, phosphorus or antimony) at the back surface and edge. As a result of diffusion and convection in the reactor, these elements can reach the wafer front surface, where they can be radially inhomogenously incorporated into the epitaxial layer, a phenomenon referred to as autodoping. Autodoping leads to a radial inhomogeneity of the electrical resistivity of the epitaxial layer.
The front and back surface of a semiconductor wafer should be defined at this point. The front surface of the semiconductor wafer is the side which is epitaxially coated and is intended for the fabrication of electronic components.
The prior art has disclosed various processes which counteract autodoping. Thus, by depositing, for example, an oxide layer or a monocrystalline or polycrystalline or amorphous protective layer on the back surface of the semiconductor wafer, it is possible to prevent the outdiffusion of substrate dopant during the epitaxy process.
Drawbacks of the known processes for producing an epitaxially coated semiconductor wafer with a protective layer for suppressing autodoping are the additional process steps which have to be carried out in different reactors, treatment baths and polishing lines. Furthermore, coating with oxide or polycrystalline semiconductor material leads to increased metal contamination.
Therefore, a number of attempts have already been proposed with a view to avoiding autodoping without the wafer back surface being provided with a protective layer prior to the deposition of the epitaxial layer.
WO 01/86034 A2 and WO 01/86035 A1, for example, have disclosed a process for the epitaxial coating of a substrate in wafer form in which the back surface of the substrate does not, as is customary, rest on a susceptor over its entire surface, but rather is exposed to a purge gas, for example hydrogen. The purge gas may be identical to or different than the front surface process gas. Dopant atoms which have diffused out via the wafer back surface are, at least in part, conveyed away by the purge gas. As a result, the proportion of the dopant atoms which diffuse past the edge of the wafer to the wafer front surface and therefore the risk of autodoping are reduced.
A similar process, although not with a view to avoiding autodoping, is described in U.S. Pat. No. 5,960,555. In this case, the object is to counteract deposition of semiconductor material on the wafer back surface. For this purpose, the wafer back surface is purged with a gas which is different than the process gas. The excess pressure prevailing on the back surface leads to the formation of a gentle flow of back surface gas past the wafer edge toward the front surface. This prevents the front surface process gas from penetrating into the back surface space and therefore deposition of semiconductor material on the wafer back surface. Back surface gases used may be inert gases, for example argon, nonreactive gases, such as nitrogen, but also gases such as hydrogen or hydrogen chloride. According to U.S. Pat. No. 5,679,405, it is also possible to use inert gases, such as helium, freon, tetrafluoromethane or hexafluoroethane, to avoid deposition of semiconductor material on the wafer back surface. The prior art does not give any satisfactory solution to the autodoping problem.